Non-alcoholic fatty liver disease (NAFLD) is very common in obesity and type 2 diabetes and it may significantly contribute to the risks associated with these metabolic disorders. Moreover, NAFLD by itself is one of the leading causes of liver transplantation in the United States, as progressive forms of the disease lead to liver failure. Although there are a growing number of epidemiological studies implicating environmental factors in the etiology of NAFLD, mechanistic studies of how these factors are integrated into liver lipid regulation have been lacking. The proposed research will focus on two important regulatory pathways in the liver controlled by histone deacetylase 3 (HDAC3) and SREBP cleavage-activating protein (SCAP). Together, these proteins control numerous facets of liver lipid metabolism including de novo lipogenesis, fatty acid oxidation, fatty acid uptake, and lipid storage. As a Class I histone deacetylase, HDAC3 functions as a transcriptional repressor of liver lipid synthesis and an epigenomic mediator of circadian signaling. SCAP on the other hand controls the activity of SREBPs, which act as potent transcription factors downstream of insulin signaling in the liver. Aside from their opposing roles in transcription, recent studies have mapped the activities of HDAC3 and SCAP to different parts of the day. Whereas HDAC3 functions to silence lipogenic gene expression during periods of sleep, SCAP/SREBP signaling is activated by feeding and nutrient-dependent cues. I hypothesize that HDAC3 and SCAP mediated pathways to fatty liver formation are distinct. To study this, I have generated double floxed HDAC3/SCAP mice to allow for conditional and liver specific knockout of both factors in adult mice. My preliminary data indicate that SREBP is not required for increased lipid accumulation in HDAC3 null livers, but unexpectedly protects the mouse from metabolic collapse. My first specific aim is to fully characterize the metabolic phenotype of mice lacking hepatic HDAC3 and SCAP. My second specific aim will focus on the downstream molecular pathways that are disrupted in HDAC3/SCAP double knockout mice and how environmental cues are integrated through HDAC3 and SCAP to regulate liver lipid metabolism. These studies will uncover basic mechanisms of hepatic lipid regulation and will inform future therapeutics to impede the epidemic of NAFLD.